The present invention relates to methods for treating pain. In particular, the present invention relates to methods for treating pain by intraspinal administration of a neurotoxin.
Many, if not most ailments of the body cause pain. Generally pain is experienced when the free nerve endings which constitute the pain receptors in the skin as well as in certain internal tissues are subjected to mechanical, thermal or chemical stimuli. The pain receptors transmit signals along afferent neurons into the central nervous system and thence to the brain.
The causes of pain can include inflammation, injury, disease, muscle spasm and the onset of a neuropathic event or syndrome. Ineffectively treated pain can be devastating to the person experiencing it by limiting function, reducing mobility, complicating sleep, and dramatically interfering with the quality of life.
Inflammatory pain can occur when tissue is damaged, as can result from surgery or due to an adverse physical, chemical or thermal event or to infection by a biologic agent. Although inflammatory pain is generally reversible and subsides when the injured tissue has been repaired or the pain inducing stimulus removed, present methods for treating inflammatory pain have many drawbacks and deficiencies. Thus, the typical oral, parenteral or topical administration of an analgesic drug to treat the symptoms of pain or of, for example, an antibiotic to treat inflammatory pain causation factors can result in widespread systemic distribution of the drug and undesirable side effects. Additionally, current therapy for inflammatory pain suffers from short drug efficacy durations which necessitate frequent drug readministration with possible resulting drug resistance, antibody development and/or drug dependence and addiction, all of which are unsatisfactory. Furthermore, frequent drug administration increases the expense of the regimen to the patient and can require the patient to remember to adhere to a dosing schedule.
Neuropathic pain is a persistent or chronic pain syndrome that can result from damage to the nervous system, the peripheral nerves, the dorsal root ganglion or dorsal root, or to the central nervous system. Neuropathic pain syndromes include allodynia, various neuralgias such as post herpetic neuralgia and trigeminal neuralgia, phantom pain, and complex regional pain syndromes, such as reflex sympathetic dystrophy and causalgia. Causalgia is characterized by spontaneous burning pain combined with hyperalgesia and allodynia.
Unfortunately, current methods to treat neuropathic pain, such as by local anesthetic blocks targeted to trigger points, peripheral nerves, plexi, dorsal roots, and to the sympathetic nervous system have only short-lived antinociceptive effects. Additionally, longer lasting analgesic treatment methods, such as blocks by phenol injection or cryotherapy raise a considerable risk of irreversible functional impairment. Furthermore, chronic epidural or intrathecal (collectively "intraspinal") administration of drugs such as clonidine, steroids, opioids or midazolam have significant side effects and questionable efficacy.
Tragically there is no existing method for adequately, predictably and specifically treating established neuropathic pain (Woolf C. et al., Neuropathic Pain: Aetiology, Symptoms, Mechanisms, and Management, Lancet 1999; 353: 1959-64) as present treatment methods for neuropathic pain consists of merely trying to help the patient cope through psychological or occupational therapy, rather than by reducing or eliminating the pain experienced.
Spasticity or muscle spasm can be a serious complication of trauma to the spinal cord or other disorders that create damage within the spinal cord and the muscle spasm is often accompanied by pain. The pain experienced during a muscle spasm can result from the direct effect of the muscle spasm stimulating mechanosensitive pain receptors or from the indirect effect of the spasm compressing blood vessels and causing ischemia. Since the spasm increases the rate of metabolism in the affected muscle tissue, the relative ischemia becomes greater creating thereby conditions for the release of pain inducing substances.
Within the enclosure by the vertebral canal for the spinal cord by the bones of the vertebrae, the spinal cord is surrounded by three meningeal sheaths which are continuous with those which encapsulate the brain. The outermost of these three meningeal sheaths is the dura matter, a dense, fibrous membrane which anteriorally is separated from the periosteum of the vertebral by the epidural space. Posterior to the dura matter is the subdural space. The subdural space surrounds the second of the three meningeal sheaths which surround the spinal cord, the arachnoid membrane. The arachnoid membrane is separated from the third meningeal sheath, the pia mater, by the subarachnoid or intrathecal space. The subarachnoid space is filled with cerebrospinal fluid (CSF). Underlying the pia mater is the spinal cord. Thus the progression proceeding inwards or in posterior manner from the vertebra is the epidural space, dura mater, subdural space, arachnoid membrane, intrathecal space, pia matter and spinal cord.
Therapeutic administration of certain drugs intraspinally, that is to either the epidural space or to the intrathecal space, is known. Administration of a drug directly to the intrathecal space can be by either spinal tap injection or by catheterization. Intrathecal drug administration can avoid the inactivation of some drugs when taken orally as well and the systemic effects of oral or intravenous administration. Additionally, intrathecal administration permits use of an effective dose which is only a fraction of the effective dose required by oral or parenteral administration. Furthermore, the intrathecal space is generally wide enough to accommodate a small catheter, thereby enabling chronic drug delivery systems. Thus, it is known to treat spasticity by intrathecal administration of baclofen. Additionally, it is known to combine intrathecal administration of baclofen with intramuscular injections of botulinum toxin for the adjunct effect of intramuscular botulinum for reduced muscle spasticity. Furthermore, it is known to treat pain by intraspinal administration of the opioids morphine and fentanyl, as set forth in Gianno, J., et al., Intrathecal Drug Therapy for Spasticity and Pain, Springer-Verlag (1996), the contents of which publication are incorporated herein by reference in its entirety.
The current method for intrathecal treatment of chronic pain is by use of an intrathecal pump, such as the SynchroMed.RTM. Infusion System, a programmable, implanted pump available from Medtronic, Inc., of Minneapolis, Minnesota. A pump is required because the antinociceptive or antispasmodic drugs in current use have a short duration of activity and must therefore be frequently readministered, which readministration is not practically carried out by daily spinal tap injections. The pump is surgically placed under the skin of the patient's abdomen. One end of a catheter is connected to the pump, and the other end of the catheter is threaded into a CSF filled subarachnoid or intrathecal space in the patient's spinal cord. The implanted pump can be programmed for continuous or intermittent infusion of the drug through the intrathecally located catheter. Complications can arise due the required surgical implantation procedure and the known intrathecally administered drugs for pain have the disadvantages of short duration of activity, lipid solubility which permits passage out of the intrathecal space and systemic transport and/or diffusion to higher CNS areas with potential respiratory depression resulting.
Thus, a significant problem with many if not all of the known intrathecally administered drugs used to treat pain, whether administered by spinal tap or by catheterization, is that due to the drug's solubility characteristics, the drug can leave the intrathecal space and additionally due to poor neuronal binding characteristics, the drug can circulate within the CSF to cranial areas of the CNS where brain functions can potentially be affected.
Botulinum Toxin
The anaerobic, gram positive bacterium Clostridium botulinum produces a potent polypeptide neurotoxin, botulinum toxin, which causes a neuroparalytic illness in humans and animals referred to as botulism. The spores of Clostridium botulinum are found in soil and can grow in improperly sterilized and sealed food containers of home based canneries, which are the cause of many of the cases of botulism. The effects of botulism typically appear 18 to 36 hours after eating the foodstuffs infected with a Clostridium botulinum culture or spores. The botulinum toxin can apparently pass unattenuated through the lining of the gut and attack the central nervous system. The highest cranial nerves are affected first, followed by the lower cranial nerves and then the peripheral motor neurons. Symptoms of untreated botulinum toxin poisoning can progress from and include medial rectus paresis, ptosis, sluggish pupillary response to light, difficulty walking, swallowing, and speaking, paralysis of the respiratory muscles and death.
Botulinum toxin type A is the most lethal natural biological agent known to man. It has been determined that 39 units per kilogram (U/kg) of intramuscular BOTOX.RTM..sup.1 is a LD.sub.50 in primates. One unit (U) of botulinum toxin can be defined as the LD.sub.50 upon intraperitoneal injection into mice. BOTOX.RTM. contains about 4.8 ng of botulinum toxin type A complex per 100 unit vial. Thus, for a 70 kg human a LD.sub.50 of about 40 U/kg would be about 134 ng or 28 vials (2800 units) of intramuscular BOTOX.RTM.. Seven immunologically distinct botulinum neurotoxins have been characterized, being respectively neurotoxin serotypes A, B, C1, D, E, F and G each of which is distinguished by neutralization with type-specific antibodies. The neurotoxin component is noncovalently bound to nontoxic proteins to form high molecular weight complexes. The different serotypes of botulinum toxin vary in the animal species that they affect and in the severity and duration of the paralysis they evoke. For example, it has been determined that botulinum toxin type A is 500 times more potent, as measured by the rate of paralysis produced in the rat, than is botulinum toxin type B. Additionally, botulinum toxin type B has been determined to be non-toxic in primates at a dose of 480 U/kg which is about 12 times the primate LD.sub.50 for botulinum toxin type A (Moyer E et al., Botulinum Toxin Type B: Experimental and Clinical Experience, being chapter 6, pages 71-85 of "Therapy With Botulinum Toxin", edited by Jankovic, J. et al. (1994), Marcel Dekker, Inc.)
 FNT .sup.1 botulinum toxin type A purified neurotoxin complex, available from Allergan, Inc., of Irvine, California. A botulinum toxin type A complex is also available from Porton Products, Ltd., U.K. under the trade name DYSPORT)
Minute quantities of botulinum toxin have been used to reduce excess skeletal and smooth muscle and sphincter contraction. The botulinum toxin can be injected directly into the hyperactive or hypertonic muscle or its immediate vicinity and is believed to exert its effect by entering peripheral, presynaptic nerve terminals at the neuromuscular junction and blocking the release of acetylcholine. The affected nerve terminals are thereby inhibited from stimulating muscle contraction, resulting in a reduction of muscle tone. Thus, when injected intramuscularly at therapeutic doses, botulinum toxin type A can be used to produce a localized chemical denervation and hence a localized weakening or paralysis and relief from excessive involuntary muscle contractions.
Clinical effects of peripheral intramuscular botulinum toxin type A are usually seen within one week of injection. The typical duration of symptomatic relief from a single intramuscular injection of botulinum toxin type A averages about three months.
Muscles therapeutically treated with a botulinum toxin eventually recover from the temporary paralysis induced by the toxin, due possibly to the development of new nerve sprouts or to reoccurrence of neurotransmission from the original synapse, or both. A nerve sprout may establishes a new neuromuscular junction. Thus, neuromuscular transmission can gradually return to normal over a period of several months.
In skeletal and smooth muscle tissues botulinum toxin appears to have no appreciable affinity for organs or tissues other than cholinergic neurons at the neuromuscular junction where the toxin binds to and is internalized by neuronal receptors and, as indicated, block presynaptic release of the neurotransmitter acetylcholine, without causing neuronal cell death.
Botulinum toxins have been used for the treatment of an increasing array of disorders, relating to cholinergic nervous system transmission, characterized, for example, by hyperactive neuromuscular activity in specific focal or segmental striated or smooth muscle regions. Thus, intramuscular injection of one or more of the botulinum toxin serotypes has been used to treat, blepharospasm, spasmodic torticollis, hemifacial spasm, spasmodic dysphonia, oral mandibular dystonia and limb dystonias, myofacial pain, bruxism, achalasia, trembling chin, spasticity, juvenile cerebral palsy, hyperhydrosis, excess salivation, non-dystonic tremors, brow furrows, focal dystonias, tension headache, migraine headache and lower back pain. Not infrequently, a significant amount of pain relief has also been experienced by such intramuscular therapy. These benefits have been observed after local intramuscular injection of, most commonly botulinum toxin type A, or one or another of the other botulinum neurotoxin serotypes. Botulinum toxin serotypes B, C1, D, E and F apparently have a lower potency and/or a shorter duration of activity as compared to botulinum toxin type A at a similar dosage level.
Although all botulinum toxins serotypes apparently inhibit release of the neurotransmitter acetylcholine at the neuromuscular junction, they do so by affecting different neurosecretory proteins and/or cleaving these proteins at different sites. For example, botulinum types A and E both cleave the 25 kiloDalton (kD) synaptosomal associated protein (SNAP-25), but they target different amino acid sequences within this protein. Botulinum toxin types B, D, F and G act on vesicle-associated protein (VAMP, also called synaptobrevin), with each serotype cleaving the protein at a different site. Finally, botulinum toxin type C1 has been shown to cleave both syntaxin and SNAP-25. These differences in mechanism of action may affect the relative potency and/or duration of action of the various botulinum toxin serotypes.
The molecular weight of a secreted botulinum toxin protein molecule, for all seven of the known botulinum toxin serotypes, is about 150 kD. Interestingly, the botulinum toxins are released by Clostridial bacterium as complexes comprising the 150 kD botulinum toxin protein molecule along with associated non-toxin proteins. Thus, the botulinum toxin type A complex can be produced by Clostridial bacterium as 900 kD, 500 kD and 300 kD forms. Botulinum toxin types B and C1 is apparently produced as only a 500 kD complex. Botulinum toxin type D is produced as both 300 kD and 500 kD complexes. Finally, botulinum toxin types E and F are produced as only approximately 300 kD complexes. The complexes (i.e. molecular weight greater than about 150 kD) are believed to contain a non-toxin hemaglutinin protein and a non-toxin and non-toxic nonhemaglutinin protein. These two non-toxin proteins (which along with the botulinum toxin molecule comprise the relevant neurotoxin complex) may act to provide stability against denaturation to the botulinum toxin molecule and protection against digestive acids when toxin is ingested. Additionally, it is possible that the larger (greater than about 150 kD molecular weight) botulinum toxin complexes may result in a slower rate of diffusion of the botulinum toxin away from a site of intramuscular injection of a botulinum toxin complex.
The biochemical mechanism of the effects of botulinum toxin upon central nervous tissues is controversial. Additionally, the number of CNS neurotransmitters affected as well as the extent and nature of the effect of botulinum toxin upon the synthesis, release, accumulation and metabolism of different CNS neurotransmitters is still being determined. In vitro studies have indicated that botulinum toxin inhibits potassium cation induced release of both acetylcholine and norepinephrine from primary cell cultures of brain tissue. Additionally, it has been reported that botulinum toxin inhibits the evoked release of both glycine and glutamate in primary cultures of spinal cord neurons and that in brain synaptosome preparations botulinum toxin inhibits the release of each of the neurotransmitters acetylcholine, dopamine, norepinephrine, CGRP and glutamate.
Botulinum toxin type A can be obtained by establishing and growing cultures of Clostridium botulinum in a fermenter and then harvesting and purifying the fermented mixture in accordance with known procedures. All the botulinum toxin serotypes are initially synthesized as inactive single chain proteins which must be cleaved or nicked by proteases to become neuroactive. The bacterial strains that make botulinum toxin serotypes A and G possess endogenous proteases and serotypes A and G can therefore be recovered from bacterial cultures in predominantly their active form. In contrast, botulinum toxin serotypes C1, D and E are synthesized by nonproteolytic strains and are therefore typically unactivated when recovered from culture. Serotypes B and F are produced by both proteolytic and nonproteolytic strains and therefore can be recovered in either the active or inactive form. However, even the proteolytic strains that produce, for example, the botulinum toxin type B serotype only cleave a portion of the toxin produced. The exact proportion of nicked to unnicked molecules depends on the length of incubation and the temperature of the culture. Therefore, a certain percentage of any preparation of, for example, the botulinum toxin type B toxin is likely to be inactive, possibly accounting for the known significantly lower potency of botulinum toxin type B as compared to botulinum toxin type A. The presence of inactive botulinum toxin molecules in a clinical preparation will contribute to the overall protein load of the preparation, which has been linked to increased antigenicity, without contributing to its clinical efficacy. Additionally, it is known that botulinum toxin type B has, upon intramuscular injection, a shorter duration of activity and is also less potent than botulinum toxin type A at the same dose level.
What is needed therefore is a method for effectively treating pain and/or spasm by intraspinal administration of a pharmaceutical which has the characteristics of long duration of activity, low rates of diffusion out of an intrathecal space where administered, low rates of diffusion to other intrathecal areas outside of the site of administration, specificity for the treatment of pain and limited or insignificant side effects at therapeutic dose levels.